Magnetic tape media having a so-called serpentine track format and high track densities employ track-following servo systems for ensuring faithful track to head gap registration laterally along elongated data tracks. It is well known that tape media are subject to media defects at which no data can be successfully recorded. For ensuring that the head gap to data track registration is maintained, it is important that the servo tracks be successfully read. If certain read errors occur while reading the servo tracks, then, to avoid unintended overwriting data recorded on adjacent tracks, writing data onto data tracks must be aborted as soon as such servo read errors are detected. It is desired that such servo track defects be identified on the media such that later recording in the same area of the tape can be controlled to avoid recording data adjacent to said servo track defect. This defect avoidance, eliminates unintended reading invalid (old) data that now, because of an adjacent servo error, can never be overwritten. Storing such data in an error log saves such error data for later avoiding unintended writing and reading.
When encountering such defects in a servo area of the medium, a choice is made to "cross out" the defect from the medium such that data processing (recording and reading) may be temporarily suspended while scanning in a lateral alignment to such detected defective servo tracks. In NRZI 9-track recording, a so-called erase gap was created that extended over track defects. Such an erase gap appeared as an elongated inter-block gap (IBG). This prior art error indication is useful only if one could still write the data obliterated by a media defect. Therefore, in servo track controlled writing wherein writing may not be an available error recovery, it is desired to provide a more reliable indication of tape media defects. All blocks of data recorded on tape are addressable by a block ID and by a tachometer count value.